# LM1875 on a single supply

M

#### [email protected]

Jan 1, 1970
0
I was reading the datasheet for the LM1875
http://www.national.com/ds.cgi/LM/LM1875.pdf

and I noticed that they do have a schematic for a single supply (as
opposed to a +/- supply).

If running on a single supply, does the chip amplifer consume
significant power during quiet tracks?

Would this be pretty much the same as running the amplifier in Class A
mode (vs. Class AB)?

I'm trying to figure out if I really need a +/- power supply, or if I
can just go with a single supply (easier to build).

Thanks,

Michael

M

#### [email protected]

Jan 1, 1970
0
The datasheet does have a "Typical Single Supply Operation" schematic.
Have another look.

Yes, that's what I said. There is a schematic. It's on Page 2.
(amused grin)

Michael

M

#### [email protected]

Jan 1, 1970
0
---
Look at "Supply Current" on page 2 and read: "Power Dissipation and
Heat Sinking" on page 6.

JF

Ah, so apparently the chip does not use more power with a single
supply than with a +/- supply.

Is there any reason why I should NOT use a single supply with this
chip?

Thanks,

Michael

M

#### [email protected]

Jan 1, 1970
0
---
I don't think so, but try it and see if it works the way you want it
to. If it does you've saved a supply, but if it doesn't it's only
cost you a handful of discretes.

JF

Good point. Thanks.

Michael

P

#### Paul E. Schoen

Jan 1, 1970
0
I was reading the datasheet for the LM1875
http://www.national.com/ds.cgi/LM/LM1875.pdf

and I noticed that they do have a schematic for a single supply (as
opposed to a +/- supply).

If running on a single supply, does the chip amplifer consume
significant power during quiet tracks?

Would this be pretty much the same as running the amplifier in Class A
mode (vs. Class AB)?

I'm trying to figure out if I really need a +/- power supply, or if I
can just go with a single supply (easier to build).

Thanks,

Michael

Since the LM1875 is inexpensive, you might try using two of them on a
single supply, and putting the speaker between the outputs. You would have
to make one amp inverting and the other non-inverting, but you will get
twice the output voltage and save the capacitors. A single 24 volt supply
will then give +/- 24 volts, or about 17 VRMS, to an 8 ohm speaker, or 36
watts. There are lots of 24 VDC supplies available. The downside of this
approach is making sure the DC level of both amps are equal and about 1/2
the supply.

If you check SemiElectric's eBay store you will find PC boards and kits for
the LM3875, as well as other single and dual amplifier ICs:

http://search.stores.ebay.com/semie...ooZ2QQfsopZ3QQfsubZ0QQsaselZ650588702QQsofpZ0

His LM3875s are about $6.50 each. They are larger (56 watts vs 20 watts), but seem to be otherwise quite similar. Most of the additional IC pins are NCs, for heat sinking and mechanical mounting, I suppose. They are only$5.90 each from DigiKey, and the LM1875 is $3.28. Wow: 10 to 15 cents per watt! Paul M #### [email protected] Jan 1, 1970 0 Since the LM1875 is inexpensive, you might try using two of them on a single supply, and putting the speaker between the outputs. You would have to make one amp inverting and the other non-inverting, but you will get twice the output voltage and save the capacitors. A single 24 volt supply will then give +/- 24 volts, or about 17 VRMS, to an 8 ohm speaker, or 36 watts. There are lots of 24 VDC supplies available. The downside of this approach is making sure the DC level of both amps are equal and about 1/2 the supply. If you check SemiElectric's eBay store you will find PC boards and kits for the LM3875, as well as other single and dual amplifier ICs: http://search.stores.ebay.com/semielectric_audio-amp_W0QQfciZQ2d1QQfc... His LM3875s are about$6.50 each. They are larger (56 watts vs 20 watts),
but seem to be otherwise quite similar. Most of the additional IC pins are
NCs, for heat sinking and mechanical mounting, I suppose. They are only
$5.90 each from DigiKey, and the LM1875 is$3.28. Wow: 10 to 15 cents per
watt!

Paul

That's a good idea. Thanks.

Michael

J

#### Jasen Betts

Jan 1, 1970
0
I would like to point out that the single supply schematic has a 2200 uF
capacitor in series with the output speaker. In theory, with an ideal
capacitor, this is going to give an 18 Hz cutoff frequency into a 4 ohm
speaker (36 Hz into 8 ohms). A real capacitor will have some internal
resistance. This is going to give a higher cutoff frequency.

Higher? I would have thunk lower.

Bye.
Jasen